The invention generally relates to a test load usable as an internal load for an indicating instrument such as a RF watt meter used for testing low power RF sources. More particularly, the invention relates to a low power RF test load and the method of application as an internal test load or dummy load which is adapted to absorb the RF energy applied thereto and effectively dissipate generated heat in an efficient manner.
In the use of the various RF transmitters, transceivers, amplifiers or other RF sources, the knowledge of the power output of the RF source will be necessary for proper coupling to an RF load such as an antenna. For example, a wattage measurement of the RF transmitter, transceiver or an RF amplifier may be required to give an indication of the incident RF energy applied to the RF load, to which the RF source is to be coupled. When testing an RF source for power output, the normal method entails coupling and operation of the RF source into a known load of proper impedance. The power into this load may be measured to provide the user with knowledge of the RF source characteristics. As an example, the RF load may ultimately be an antenna, but it may be desired to avoid unnecessary RF radiation and possible interference which can be accomplished by using a shielded resistive test load in the testing procedure. The test load is matched to the transmitter such that no power will be reflected back along the coupling RF transmission line and all power will be absorbed by the test load. In this manner, the test load enables a true representation of power to be measured by a suitable RF watt meter as an example.
For high powered devices, the test load would necessarily need to dissipate hundreds of watts, wherein the construction of the test load may become quite complex and expensive in its manufacture to achieve such dissipation. For smaller RF sources, such as smaller transmitters used in mobile telephones, hand-held and mobile two-way radios, or other smaller RF sources conventionally used in industry, the test load will also need to dissipate RF power, although significantly less power than with high powered devices. In high powered applications, the test load will normally be an external load wherein the heat may be properly dissipated without affecting or damaging the testing or measuring device. For lower RF power devices, such as 50 watts or less, the test load may be incorporated into the testing instrument itself as an internal load, although various precautions must be taken with such internal loads.
Using a watt meter as an example, various internal test loads have been developed in the prior art, which may be incorporated into the watt meter itself. The internal test load in the testing instrument allows the power of the RF source to be checked without using the RF load to which the source is to be coupled. For example, the power output of a transmitter may be determined without using the transmission system antenna. An antenna may not be properly matched to the transmitter, which would cause error in any power measurements and therefore is desired to be selectively removed from the system for testing purposes. An internal test load in a watt meter also allows testing to be accomplished without having to carry or connect any additional load or other piece of test equipment so as to make testing convenient. A watt meter with an internal test load may be used to initially perform a power test, after which the watt meter may be used in a normal fashion to read the forward and reflected power in an antenna system for tuning or matching thereof.
It should be apparent that even with low power devices, an internal test load adapted to absorb all power from the RF sources will quickly become very hot, and thus prior art test loads have usually resorted to various structures to absorb and transfer heat away from the load so as to be dissipated without damaging the testing equipment. The mounting structures for the internal test load found in the prior art have normally resorted to fin-type structures or plastic standoff arrangements which constituted specialized mounting hardware. Such structures are many times very large and heavy, thereby making the measuring device in which the test load is used cumbersome and unsuitable for many applications. Similarly, other prior art methods have resorted to extremely expensive methods of dissipating generated heat from the test loads such as special machined beryllium supporting structures or the like. In yet other known configurations, the test load may be supported by styrofoam mounting techniques which tend to insulate the internal electronics of the measuring device, but result in a structure which is not extremely durable and is susceptible to being damaged by excessive heat.